How to keep a child busy for hours in the summer: DIY: Giant Ice Cube Awesomeness - filled with plastic animals, dinos, and such. And I'm sure you could put in food dye before its frozen to color it like the ice block in the photo.
Fun project to do with the kids! Melted bead suncatchers - *Layer cheap plastic beads in cake pans (no lining required), melt at 400 degrees for 20 minutes. Let cool & then flip them out. Drill a hole in it to make it a suncatcher.